---
title: "Why Do Injection Molds Develop Sink Marks? Root Causes & Fixes for Manufacturers - JATERSON"
description: "Global manufacturers face costly injection molding defects like sink marks daily. These flaws stem from material, process, and design variables that undermine part quality. Zhejiang-based injection molding specialists share actionable diagnosis, correction, and prevention strategies to reduce waste and meet production deadlines."
url: "https://www.ok-tool.com/manufacturing/why-injection-molds-develop-sink-marks-root-causes-fixes.html"
language: "en"
type: "Article"
category: "Injection Molding Guide"
datePublished: "2026-09-29"
dateModified: "2026-09-29"
brand: "JATERSON"
manufacturer: "JATERSON"
image: "https://static.ok-tool.com/uploads/industry/injection/RGqhJOkirvis0.webp"
---

# Why Do Injection Molds Develop Sink Marks? Root Causes & Fixes for Manufacturers

Sink marks are one of the most common and costly defects in injection molding,leading to part rejection,rework,and delayed shipments for manufacturers worldwide.When diagnosing why an injection mold produces sink marks,three core variables determine the outcome—ordered by their ease of adjustment and immediate impact: **process parameters**,**material properties**,and **part design**.Process parameters are the first place to investigate because they can be modified without costly tooling changes or material swaps,making them the most accessible solution for production teams.Material properties come next,as different resins behave differently during cooling and contraction.Finally,part design issues are the most challenging to address,as they often require mold modifications or redesigns,which carry higher costs and longer lead times.

## 1.Process Parameters: The Top Culprit in Day-to-Day Production

![Why Do Injection Molds Develop Sink Marks? Root Causes & Fixes for Manufacturers](https://static.ok-tool.com/uploads/industry/injection/RGqhJOkirvis0.webp)

Most sink marks in mass production stem from suboptimal process settings,especially related to how the molten plastic is packed and cooled.When plastic cools,it contracts—if the mold cavity isn’t properly filled with additional material to compensate for this contraction,a sink mark forms on the part’s surface.

### Packing Pressure and Time

The single most critical process factor is packing pressure and time.Packing pressure pushes additional molten plastic into the mold cavity as the initial shot begins to cool and contract.If the pressure is too low,or the time is too short,there isn’t enough material to fill the void created by contraction.At JATERSON,we frequently see cases where operators reduce packing time by 10-15% to meet tight lead time requests,which directly leads to sink marks in thick-walled components.To fix this,we recommend gradually increasing packing pressure by 5-10% increments (up to the machine’s safe limit) and extending packing time until the sink mark disappears.It’s important to monitor for flash (excess plastic) as a sign that pressure is too high.

### Melt Temperature

High melt temperatures increase the plastic’s viscosity,making it harder to pack into the mold cavity effectively.Overheated plastic also takes longer to cool,leading to greater contraction.For semi-crystalline materials like polypropylene (PP),excessive melt temperature can exacerbate crystallization,which causes more significant volume shrinkage.The fix here is to reduce the melt temperature by 10-15°C (18-27°F) while ensuring the plastic still flows smoothly into all sections of the mold.Always refer to the resin manufacturer’s recommended temperature range to avoid material degradation.

### Cooling Time and Uniformity

Inadequate or uneven cooling is another key cause.If the mold cools too quickly in some areas and too slowly in others,the plastic contracts unevenly,creating sink marks in the slower-cooling sections (usually thick walls).For example,a part with a 6mm wall section next to a 2mm wall will have a longer cooling time in the thicker area,leading to contraction that can’t be compensated by packing.To address this,ensure cooling channels are evenly distributed around thick sections,and extend cooling time by 10-20% to allow the plastic to solidify more uniformly.Using mold temperature controllers to maintain consistent cavity temperatures can also reduce sink mark risk.

## 2.Material Properties: How Resin Type Influences Sink Marks

Every plastic resin has unique contraction rates and cooling behaviors,which directly impact sink mark formation.Understanding these properties is critical for selecting the right material and adjusting processes accordingly.

![JATERSON Explains: Why Injection Mold Sink Marks Happen & How to Avoid Them](https://static.ok-tool.com/uploads/industry/default/rCHTMgm9r0ft5.webp)

Semi-crystalline materials (such as PP,PE,and PA) have higher contraction rates (typically 1-3%) compared to amorphous materials (ABS,PC,PS) which contract 0.5-1.5%.This higher contraction means semi-crystalline resins are more prone to sink marks,especially in thick-walled parts.Filled plastics,like glass-filled nylon,have lower contraction rates but can still develop sink marks if the filler isn’t evenly distributed or if packing parameters aren’t adjusted to account for the material’s reduced flowability.

| Material Type | Sink Mark Susceptibility | Key Mitigation Tips |
| --- | --- | --- |
| Semi-crystalline (PP,PE,PA) | High | Increase packing pressure by 10-15%; use nucleating agents to speed crystallization; optimize cooling channel placement around thick sections |
| Amorphous (ABS,PC,PS) | Medium to Low | Reduce melt temperature by 10-15°C; extend cooling time by 10-20%; ensure uniform wall thickness across the part |
| Filled Plastics (Glass-filled PA,Carbon-filled PP) | Medium | Use larger gate sizes to improve flow; increase packing time by 20%; avoid sharp transitions between thick and thin walls |

A common mistake we see at JATERSON is customers specifying a semi-crystalline resin for a part with thick walls without considering sink mark risk.In these cases,we often recommend switching to a lower-contraction amorphous resin (if functional requirements allow) or adding a nucleating agent to the semi-crystalline material to reduce contraction and speed up cooling.

## 3.Part Design: The Hidden Cause of Persistent Sink Marks

Even with optimal process parameters and the right material,poor part design can lead to unavoidable sink marks.Design-related issues are the hardest to fix because they usually require mold modifications or part redesigns,which can add weeks to lead times and increase costs.

### Thick Wall Sections

The most common design flaw is thick wall sections (typically over 4mm for most resins).Thick walls take longer to cool,leading to greater contraction that packing pressure can’t fully compensate for.For example,a tool handle with an 8mm thick grip section will almost always develop sink marks on the opposite side of the gate unless the design is modified.The solution here is to reduce wall thickness to a uniform 2-3mm where possible,or add internal ribs to maintain structural integrity without increasing overall wall thickness.Ribs should be no more than 60% of the adjacent wall thickness to avoid sink marks on the outer surface.

### Abrupt Wall Transitions

Sharp transitions between thick and thin walls cause uneven cooling and contraction,leading to sink marks in the thicker area.Instead,use gradual transitions with a taper of at least 1:5 (thickness change to length of transition) to allow the plastic to flow and cool uniformly.This helps reduce stress concentrations and sink mark risk.

### Poor Gate Placement

Gates are the entry points for molten plastic into the mold cavity.If a gate is placed too far from a thick wall section,the plastic will begin to cool before it can be packed effectively into that area,leading to sink marks.Optimal gate placement is as close as possible to thick sections to ensure adequate packing pressure reaches those areas.For example,a part with a thick boss should have a gate directly adjacent to the boss to maximize packing efficiency.

## 4.Step-by-Step Sink Mark Diagnosis Checklist

When faced with sink marks,it’s critical to systematically diagnose the root cause instead of making random adjustments.Here’s a practical checklist we use at JATERSON:

- Review process logs: Check if packing pressure,time,melt temperature,or cooling time was changed before sink marks appeared.Even small adjustments (like a 5°C increase in melt temp) can cause defects.
- Inspect material batches: Confirm if the resin grade,filler content,or supplier changed.Batch-to-batch variations in resin can affect contraction rates.
- Analyze part design: Map out thick wall sections,transitions,and gate placement to identify potential design flaws.
- Conduct controlled tests: Adjust one parameter at a time (starting with packing pressure) and measure the impact on sink marks.This isolates the root cause and avoids compounding issues.
- Check mold condition: Inspect for worn mold cavities,blocked cooling channels,or damaged gates that could affect flow and cooling.

## 5.Long-Term Prevention Strategies

To reduce sink mark risk in future projects,combine process optimization,material selection,and design best practices:

- Design for manufacturability (DFM): Work with your injection molding partner early in the design phase to optimize wall thickness,transitions,and gate placement.At JATERSON,we provide free DFM reviews for OEM/ODM customers to catch design issues before mold fabrication.
- Establish standardized process parameters: Document optimal settings for each material and part design,and train operators to follow these standards.Regularly audit process logs to ensure compliance.
- Use material testing: Conduct small-scale trials with different resins or additives (like nucleating agents) to find the best balance between functional requirements and sink mark resistance.
- Implement in-line quality control: Use visual inspections or automated systems to detect sink marks early in production,allowing for quick process adjustments before large quantities are rejected.

## Conclusion: Actionable Takeaways for Reducing Sink Marks

Sink marks are avoidable with the right approach.Start by investigating process parameters—packing pressure and time are the most impactful and easiest to adjust.Next,evaluate material properties and consider switching to a lower-contraction resin or adding additives if needed.Finally,address design flaws through DFM reviews or mold modifications if defects persist.At JATERSON,we’ve helped hundreds of overseas customers reduce sink mark-related waste by up to 30% by implementing these strategies.By prioritizing systematic diagnosis and proactive prevention,manufacturers can improve part quality,reduce costs,and meet tight production deadlines.

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